The city was experiencing its worst flu season since at least 2009, Menino said, with about 700 confirmed cases of the flu, compared with 70 all of last season.
Massachusetts was one of 29 states reporting high levels of "influenza-like illness," according to the most recent weekly flu advisory issued by the Centers for Disease Control and Prevention.
The CDC said the proportion of people visiting health care providers with flu-like symptoms climbed from 2.8 percent to 5.6 percent in four weeks. By contrast, the rate peaked at only 2.2 percent during the relatively mild 2011-12 flu season.
The estimated rate of flu-related hospitalizations in the U.S. was 8.1 per 100,000 people, which is high for this time of year, according to Dr. Joe Bresee, chief of the epidemiology and prevention branch of the CDC's influenza division.
Barbara Ferrer, director of the Boston public health commission, said the emergency was declared in part to get residents' attention. She said that the 700 confirmed cases represent only those reported to the city and that thousands of other people may be ill.
Boston hospitals had counted about 1,500 emergency room visits since December by people with flu-like symptoms. Menino said people with symptoms shouldn't go to work or school.
LaKeisha Davis, 23, was at the Whittier Street Health Center on Wednesday for treatment of unrelated pain when she heard about the flu emergency being declared in Boston.
She took a flu vaccine on the spot, fearing that if she got the flu her 4-year-old daughter might catch it as well.
"I love her more than anything in the world and I don't want anything to happen to her," Davis said.
Frederica Williams, president of the community health center in the inner-city Roxbury neighborhood, said her facility had opened a special flu clinic and was using social media and sending letters to residents urging them to come in and get flu shots. Williams estimated that the number of patients coming to the clinic with flu-like symptoms was triple that of a year ago.
Hospitals around the state were also taking precautions to protect patients and staff members from exposure to the flu.
Baystate Health, which operates Baystate Medical Center in Springfield and two other hospitals in western Massachusetts, announced it was changing its visitor policy. The hospitals will no longer allow visitors younger than 14 and are recommending no more than two people visit a patient at once.
"This is the worst in several years," said Dr. Sarah Haessler, an infectious disease specialist at Baystate. She said the flu outbreak has strained the hospital's resources and helped to fill its beds to capacity.
City, state and federal officials have all identified a Type A influenza known as H3N2 as the predominant strain reported so far this season. The strain, historically associated with more serious illnesses, is among those covered by the current vaccine.
"No vaccine is 100 percent effective," cautioned Kevin Cranston, head of the state bureau of infectious diseases. Some people, for example, might be vaccinated but get the flu in the 10 days to two weeks it takes for the immunity to take hold.
"There are any number of reasons why people could have done all the right things and still get the flu," he said.
High flu rates were being reported all over Massachusetts, Cranston said, and while he didn't have specifics on the 18 statewide deaths, he noted that the flu is most dangerous for the young, the elderly and people with other chronic health conditions.
"I hate needles, and I got (a shot)," Gov. Deval Patrick said Wednesday, adding that he wasn't aware of any shortages of vaccine in the state. He also reminded residents to use common sense, such as washing their hands and sneezing into their sleeves.
The CDC said 18 children have died from the flu so far this season. While the CDC doesn't keep a tab of deaths overall from the flu, it estimates that 24,000 Americans die each year.